The present invention generally relates to female vacuum forming apparatus. In particular, the present invention relates to a female vacuum forming tool that provides improved draw depth capability for forming parts such as automotive instrument panel covers.
Existing female vacuum forming apparatus comprises a porous female cavity having small air canals, a male portion called a plug, and a clamp frame. A sheet of thermoplastic material is loaded into the clamp frame and heated to its softening point wherein it begins to sag. The sheet is then placed in between the female cavity and plug. The plug lowers to assist in pushing portions of the sheet in close proximity to the surface of the female cavity. Vacuum pressure is then used to pull the sheet against the surface of the female cavity.
In order to form a vacuum, current design standards dictate that the female cavity be enclosed and that the female portion of the tool, the male plug, and the clamp frame mate at a point near the top of the part shape. In a case where a part has a deep V-shaped cross-section, but is otherwise open on its ends, the corresponding female tool contains large sections called run-off areas which serve to enclose the tool cavity. The run-off areas are not used to form actual part shape. During the conventional forming process, portions of the sagging sheet contact these run-off areas and begin to cool more quickly than areas of the sheet not yet in contact with the female tool. As the sheet cools, the thermoplastic material loses its stretching properties. This can inhibit stretch into a deep draw section adjacent to the run-off areas, resulting in thin-walled sections or rupture of the sheet.
The present invention addresses these problems by providing a tool with increased deep draw capabilities by decreasing contact with areas of the tool that do not contain part shape. The present invention does not require that the female cavity be enclosed completely around the tool. Instead, the profile of the tool more closely conforms with the shape of the part, reducing the run-off sections and leaving open sections in the female cavity. Open sections in the tool allow the thermoplastic sheet to sag in open air without cooling against cooler run-off areas. When the tool eventually seals with the male plug and the vacuum pressure is drawn, the thermoplastic sheet can be stretched more effectively into the deep draw section.
Briefly stated, in accordance with one aspect of the present invention, a female mold vacuum forming apparatus is provided having a top contoured surface and a cavity formed within the top contoured surface. The cavity has a cavity surface which contains air canals. The cavity surface also defines part geometry having deep draw sections. The top contoured surface defines open sections in the side surfaces of the mold adjacent to at least one of the deep draw sections to allow free formation of parts within the deep draw sections.
In accordance with another aspect of the present invention, a female mold vacuum apparatus is provided comprising a cavity and a cavity surface having air canals. The cavity surface also defines part geometry having deep draw sections. An open section is defined within a side surface of the mold adjacent to at least one deep draw section to allow unhindered formation of parts within the deep draw section.
In accordance with yet another aspect of the invention, a method for vacuum forming thermoplastic parts is provided. In the first step, a female mold is provided having a cavity and cavity surface with air canals. Open sections are defined in the side surfaces of the mold to allow free formation of parts within deep draw sections. A softened thermoplastic sheet is then lowered into the cavity wherein the sheet does not contact the mold at the open sections. Air is then drawn through the air canals wherein negative pressure is created at the cavity surface.